The electrolytic production of hypochlorite solutions from sea water or diluted brine is a well-known electrochemical process. Electrochemically produced sodium or potassium hypochlorite produced in this way is used, for instance, as a bleaching agent. One of the most important applications for diluted hypochlorite solutions is in the field of water disinfection. The electrolysis process is usually carried out in monopolar or bipolar undivided cells with interleaved planar electrodes. Sea water, or a diluted brine at a typical chloride concentration of 15 to 40 grams/liter, is circulated in the electrolysis cell under a moderate current density to provide a hypochlorite solution at the anode at a typical concentration of 2 to 10 g/l.
Typical electrode materials of electrochemical hypochlorite cells are titanium or various types of steel for the hydrogen-evolving cathodes and titanium coated with noble metal-containing mixed oxide systems for the anodes. Titanium is particularly suitable for cells of bipolar configuration, wherein a bipolar plate can be obtained by coating one face of a titanium plate, to be used as the anodic face, with a suitable catalytic oxide mixture (for instance a mixture of ruthenium, iridium and titanium oxides), while the opposite, uncoated face is used as the cathodic one. One acknowledged problem with titanium or stainless steel cathode surfaces is that part of the hypochlorite product coming in contact therewith is reduced back to chloride, thereby lowering the efficiency of the process.
A similar problem is experienced in electrolysis cells for chlorate production, wherein chlorate produced at the anode also has the tendency of being partially reduced at the cathode. In the case of chlorate cells, this phenomenon is usually handled by filming the cathode with a chromium hydroxide film obtained by introducing sodium dichromate into the electrolytic bath. Such measure is not applicable, however, in hypochlorite systems for water disinfection, in which the presence of chromium is not acceptable.
Another solution that can be adopted in hypochlorite cells is to provide the titanium cathode surfaces with a titanium oxide plasma-sprayed intermediate layer, followed by a ZrO2 plasma-sprayed layer. Another option is to plate a layer of platinum catalyst on the titanium cathode as the intermediate layer, again followed by a ZrO2 plasma-sprayed layer. Zirconium oxide layers of suitable porosity can help prevent the anodic product from reaching the cathode active sites and have a beneficial effect on the overall current efficiency. Nevertheless, the operative lifetime of ZrO2-coated titanium cathodes is normally too limited to justify the costs of a plasma spray application. In fact, although ZrO2 is very stable to the caustic environment established on cathode surfaces, the cathodically-evolved hydrogen tends to detach the protective layer from the titanium body in a short time.
It is, therefore, highly desirable to identify a cathode material for use in electrochemical hypochlorite production which will allow a higher current efficiency with a suitable lifetime.